Method for the manufacturing of alkali monofluorophosphate

ABSTRACT

In a method for manufacturing alkalimonofluorophosphate of general formula M 2  PO 3  F(I), where M stands for the cation of a metal of the first main group of the periodic system, in particular potassium, a reaction mixture containing alkali metal cations M, phosphate, fluoride, particularly in the form of alkali hydroxide, phosphoric acid and hydrofluoric acid, and water, the molar ratio of M:P:F being (2±0.1):(1±0.05):(1±0.1) and the molar ratio of water:P being at least 1:1, is heated to a temperature of 150° C. to 400° C. Alkalimonofluorophosphate of general formula (I) occurs, with evaporation of water, as a reaction product, which is then isolated.

The invention relates to a method for manufacturing alkalimonofluorophosphate of the formula M₂ PO₃ F. Alkalimonofluorophosphates, such as potassium fluorophosphate, are used asfluorine carriers in toothpastes or drinking water. Alkalimonofluorophosphates also have a fungicidal and insecticidal effect andare therefore also used in wood preservatives. Furthermore, alkalimonofluorophosphates are valuable intermediate products for themanufacturing of other monofluorophosphate compounds which, for theirpart, are used in wood preservativese such as monofluorophosphatecompounds containing copper and zinc.

It is known that, for the manufacturing of alkali monofluorophosphates,anhydrous phosphates can be caused to react with fluorides at a veryhigh temperature, usually above 600° C. The resulting corrosive meltrequires corresponding equipment expenditures for the reactors.

The German Published Patent Application DE- B 1 224 280 relates to themanufacture of different metal fluorophosphates. Based on aqueous oranhydrous mixtures which contain metal kations, phosphorus and fluorineat alternating ratios, by means of the reaction at temperatures of up to800° C., reaction products are produced which contain theabove-mentioned elements. The DE-B 1 224 280 contains nothing concerningthe type of chemical compounds contained in the reaction products. Atargeted teaching for the synthesis of defined metal fluorophosphates isnot found, only information that according to the type and quantity ofthe starting materials of the reaction conditions, such as temperatureand time, many different products are obtained.

It is an object of the invention to provide a technically simple methodfor the manufacturing of defined metal fluorophosphates, specificallyalkali mono-rubidium monofluorophosphate, and cesiummonofluorophosphate. These objects are achieved by means of the methodaccording to the invention. The method according to the invention forthe manufacturing of alkali monofluorophosphate of the general formulaM₂ PO₃ F(I) is characterized in that a reaction mixture which containsalkali metal cations M, phosphate, fluoride and water--the molar ratioof M:P:F being (2±0.1):(1±0.05):(1±0.1), and the molar ratio of water:Pbeing at least 1:1 is heated to a temperature of from 150° C. to 400°C., the alkali monofluorophosphate of the general formula (I) beingformed as the reaction product, and the reaction product being isolated,if desired, after an aftertreatment.

When, for example, a reaction mixture is reacted in which M stands forsodium, according to the method of the invention, sodiummonofluorophosphate can be produced which has a purity of approximately80% in weight. The other alkali monofluorophosphates are obtained withan even higher purity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates equipment which can be used to carry out the processof the present invention.

According to a preferred embodiment of the method according of theinvention, pure alkali monofluorophosphates are manufactured. Thispreferred embodiment of the method according to the invention ischaracterized in that, for the manufacturing of a pure alkalimonofluorophosphate of the general formula M₂ PO₃ F (I), wherein Mstands for potassium, cesium or rubidium, a reaction mixture whichcontains alkali metal cations M, phosphate, fluoride and water -themolar ratio of M:P:F being (2±0.05):(1±0.05):(1±0.1), and the molarratio of water:P being at least 1:1--is heated to a temperature of from150° C. to 400° C., the alkali monofluorophosphate of the generalformula (I) being formed as the reaction product while water isevaporated, and the reaction product being isolated, if desired, afteran aftertreatment.

In the present invention, it is particularly preferred that "alkali" orM stands for potassium. The method according to the invention isparticularly well suited for the manufacturing of potassiummonofluorophosphate--K₂ PO₃ F.

When, in the following, for reasons of simplicity, "alkalimonofluorophosphate" is mentioned, this term applies to compounds of theformula (I) which should correctly be called "dialkalimonofluoromonophosphate".

Expediently, the amounts of starting compounds are selected such thatthe molar ratio of M:P:F is approximately 2:1:(1-1.1). In this manner,particularly pure alkali monofluorophosphates of the general formula (I)can be produced.

The ratio of water:P is preferably at least 2:1, for example, 2:1 to100:1. Preferably, the ratio of water:P is at least 3:1. In aparticularly preferred manner, an aqueous solution is used as thereaction mixture.

Aqueous solutions which contain approximately 11.9 to 14.7 mol of waterfor each mol of the M₂ PO₃ F end product to be manufactured areexcellently suitable for use as the reaction mixture.

When caustic potash solution, phosphoric acid and hydrofluoric acid areused as the basis, reaction mixtures are used, for example, which, inaddition to KOH, H₃ PO₄ and HF also contain 41 to 47.7% in weight ofwater. When a quantitative reaction to K₂ PO₃ F is assumed, such amixture will then still contained 11.9 to 14.7 mol of water per K₂ PO₃ Fto be manufactured.

The phosphate can be fed into the reaction mixture in many differentmanners. In principle, any compounds of the pentavalent phosphorus maybe used that can be hydrolyzed to form phosphate. Compounds areexpediently used which are constituted of pentavalent phosphorus, oxygenand possibly hydrogen and/or alkali metal or ammonium cations.Phosphorus pentoxide, phosphoric acid, dimeric, oligomeric or polymericphosphoric acid as well as corresponding monobasic, bibasic or tribasicalkali or ammonium compounds may, for example, be used. Alkalidihydrogen phosphate, dialkali monohydrogen phosphate or trialkaliphosphate may, for example, be used. Also usable are correspondingammonium compounds, also polyphosphates, including hydrogenpolyphosphates which are characterized by the general total formulaX.sub.(n+2) P_(n) O₃(n+1), in which X stands for M and/or NH₄ and/or Hand in which n≧2. Preferably in this case n is equal to 2 or 3. For aperson skilled in the art it is self-evident that polyphosphates of ahigh molecular weight which cannot be reacted at a temperature of up to400° C., such as Kurrol's potassium salt, which has a high molecularweight, cannot be used, but the water-soluble potassium polyphosphatedescribed by Klement and Schmid, Z. Anorg. Allg. Chem. 290 (1957), Pages113 to 132, may be used.

Also usable are cyclophosphates, including hydrogen cyclophosphates andcyclophosphoric acids which are characterized by the general totalformula X_(n) P_(n) O_(3n), in which X has the above-mentioned meaningand in which n≧3. In this case, n is preferably 3 or 4.

The phosphate is preferably fed into the reaction mixture in the form ofphosphoric acid, alkali dihydrogen phosphate or dialkali hydrogenphosphate, particularly preferably in the form of phosphoric acid.

The alkali metal cation may also be entered into the reaction mixture inmany different forms. Expediently, alkali compounds are used whose anionis volatile under the conditions according to the method of theinvention, such as nitrate, oxalate or acetate, or is transformed intowater in the course of the process of the invention. Alkali hydroxide,alkaline lye, alkali carbonate and alkali hydrogen carbonate areparticularly suitable. Alkali hydroxide or alkaline lye are preferablyused if M is sodium or potassium, and alkali carbonate is preferablyused when M is cesium or rubidium.

The fluoride can also be entered into the reaction mixture in manydifferent forms. It is expedient to use an alkali fluoride or an alkalibifluoride or a fluoride whose cation is volatile under the condition ofthe method of the invention, such as ammonium fluoride or ammoniumbifluoride. If an alkali fluoride or an alkali bifluoride is used, theabove-described other alkali compounds are used in such quantities thatthe ratio according to the invention of M:P:F is maintained (theanalogous situation naturally exists if alkali phosphates are used).

Preferably, the fluoride is entered into the reaction mixture in theform of hydrogen fluoride, particularly in the form of an aqueoussolution.

The water contained in the reaction mixture may be of different origins.It may, for example, be added as water; it may be fed as water ofcrystallization, water of constitution, as solvent water, for example inthe form of an aqueous phosphoric acid, as alkaline lye and/or aqueoushydrofluoric acid, together with the starting materials.

It was found to be particularly advantageous to produce the reactionmixture by mixing phosphoric acid, particularly aqueous phosphoric acid,alkaline lye and aqueous hydrogen fluoride. It is particularlyadvantageous to produce the reaction mixture by mixing phosphoric acid,particularly aqueous phosphoric acid, alkaline lye and aqueous hydrogenfluoride and to keep the reaction mixture obtained in this manner, untilthe point in time at which, as indicated above, it is heated to atemperature of 150° to 400° C., at such a temperature that no solidswill precipitate. In the case of mixtures of phosphoric acid, potassiumlye and hydrofluoric acid, for example, a minimum temperature of 60° C.was found to be advantageous. Expediently, the exothermal heatdevelopment is utilized during the mixing of the starting compounds andthe mixture is immediately reacted further.

Preferably the reaction mixture is heated to a temperature of at least200° C. Very advantageously, the reaction mixture is heated totemperature of from 220° C. to 350° C.

During the heating, water evaporates; the starting compounds react withone another; and the desired alkali monofluorophosphate is formed.

The producing of the reaction mixture and the heating may take place inthe same vessel. Expediently, vessels are used which are resistant tohydrogen fluoride, such as special-steel pots, platinum pots, aluminumvessels, or the like. However, it is difficult to remove the reactionmixture, which solidifies in the course of the reaction, from suchvessels. Expediently, the still liquid reaction mixture is thereforeheated in a flat manner.

This preferred method for the manufacturing of alkalimonofluorophosphate of the general formula M₂ PO₃ F (I), wherein Mstands for the cation of a metal of the first main group of the periodicsystem of elements, is characterized in that a reaction mixture whichcontains alkali metal cations M, phosphate, fluoride and water -themolar ratio of M:P:F being (2±0.1):(1±0.05):(1±0.1), and the molar ratioof water:P being at least 1:1--is heated in a flat manner to atemperature of from 150° C. to 400° C., the alkali monofluorophosphateof the general formula (I) being formed, while water is evaporated, asthe reaction product, and the reaction product being isolated.

For example, the reaction mixture may be produced in a vessel of anyshape that is resistant to hydrogen fluoride and this reaction mixturemay then be withdrawn onto heatable flat forms, such as metal sheets,and may be heated there. The separation of the solidified reactionproduct can then be carried out, for example, by means ofself-supporting blades, such as scrapers or knives. The use of a mixingvessel is advantageous. However, the reaction mixture may also beproduced directly on the heating form.

Technically, it is particularly advantageous to apply the reactionmixture to surfaces which are renewed continuously, for example, onto acirculating conveyor belt, such as a link conveyor, from which thesolidified reaction product which is obtained during heating isdetached, for example, by means of scrapers.

Particularly advantageously, the reaction mixture is applied to thesurface of a roller as a continuously renewable surface. In this casealso, the reaction mixture may be produced directly on the rollersurface. Expediently, the reaction mixture is first produced in a vesseland the finished mixture is then applied to the roller surface.

This particularly preferred embodiment provides that the reactionmixture, which contains phosphate, fluoride and the alkali metal cation,is applied to the surface of a roller; is heated there to a temperatureof at least 150° C., preferably 200° to 400° C., particularly preferablyto a temperature of from 220° to 350° C., and the solidified reactionproduct is detached from the roller by means of a self-supporting blade.In this case, the reaction mixture must remain on the roller surfaceuntil the reaction mixture has solidified while alkalimonofluorophosphate is formed, and a major portion of the water, forexample, more than 70%, for example, 70 to 90 or even 100% in weight ofthe existing water, is evaporated.

If desired, the product may also be subjected to a drying aftertreatmentat an increased temperature; expediently by heating it to temperaturesof at least 280° C. to 400° C., preferably 300° to 350° C. This may takeplace, for example, in corresponding furnaces or by the use ofappropriate radiation, such as infrared radiation. Surprisingly, notonly the content of water is lowered during this aftertreatment, but theproduct is also clearly improved.

The method according of the invention permits the manufacture of alkalimonofluorophosphates, particularly potassium fluorophosphate in asurprisingly pure state. The role of the water in the case of the methodaccording to the invention has so far not been clarified, but it wasfound in a plurality of experiments that a pure monofluorophosphate isobtained only when the mentioned minimum amount of water is present. Itis surprising that pure compounds can be obtained at all, because it isto be expected that water leads to the hydrolysis of the desiredfluorophosphate.

In the following, an arrangement is described by means of which a verypure monofluorophosphate can be produced in a very simple manner.

In the following examples, the method according to the invention is tobe explained in greater detail without limiting its scope.

EXAMPLES General Information

For the experiments, chemicals with a pro analysis degree of purity wereused, such as correspondingly pure phosphoric acid, caustic potashsolution, hydrofluoric acid, dipotassium hydrogen phosphate. Thesequence in which the starting compounds are mixed with one another forproducing the starting mixture is not critical.

Equipment

Examples 1 to 5 were carried out in a platinum vat. Example 6 wascarried out in equipment that is illustrated in FIG. 1. With referenceto FIG. 1, this equipment will be explained further in the following.One storage vessel 1 is used for accommodating an alkaline lye; onestorage vessel 2 for accommodating a phosphoric acid solution; and onestorage vessel 3 for accommodating a hydrofluoric acid.

By means of valves 4, 5, and 6, storage vessels 1, 2, and 3 areconnected with a mixing vessel 7. The mixing vessel 7, in turn, isconnected with the pipe 9 by means of a valve 8. On its end, the pipe 9has an opening which permits the feeding of the reaction mixture fromthe mixing vessel 7 onto an aluminum plate 10. This aluminum plate 10 isarranged to slope toward a roller 11 in such a manner that the reactionmixture which is applied to the plate 10 flows to the surface of theroller 11. The roller 11 can be rotated about a horizontal axis indirection indicated by an arrow. The reaction roller consisted of acylinder which can be heated on the inside by means of thermo-oil; has athickness of 2 cm, a length of 48 cm and an inside diameter of 38 cm.The rotating speed of the roller is expediently adjusted in such amanner that it requires approximately 20 sec. to 2 min. for onerotation. By way of the aluminum plate, the reaction mixture may beapplied along the whole width of the roller surface. By means of aself-supporting special-steel blade 12, which may be pressed against theroller by hydraulically operated presses which are not shown, the hotsolidified reaction product is detached from the roller surface. Thealuminum plate 7 and the special-metal blade 12 are arranged to bemovable. The angle between the aluminum plate 10 and the special-steelblade 12 (the roller axis is the point of intersection) expedientlyamounts to between approximately 90° and 270° C., viewed in the runningdirection of the roller. By way of the blade 12, which is expedientlyarranged to fall away from the roller, the reaction product is placedonto a conveyor belt. The conveyor belt 13 is connected with a heatingoven 14. The oven 14 is connected with the conveyor belt 15 and astorage vessel 16.

Methods Of Analysis

Method a) ¹⁹ F-NMR Spectroscopy.

By means of chemical displacement and possibly the splitting pattern(coupling with the phosphorus atom), this method of analysis permits thequantitative determination of a possible contamination of the desiredmonofluorophosphate with fluoride and difluorophosphate.

Method b) ³¹ P-NMR Spectroscopy.

By means of chemical displacement and possibly the splitting pattern(coupling with the fluorine atom or atoms), this method of analysispermits the quantitative determination of a possible contamination ofthe produced monofluorophosphate with phosphate or difluorophosphate.

Method c) Quantitative Paper-Chromatographic Analysis According toRossel.

This method permits the quantitative determination of possible otherphosphates in addition to the desired monofluorophosphate.

A description of this method is found in R. Rossel, Z. Anal. Chem. 196(1973), Pages 6 to 15. A chromatographic paper is used that was cut to alength of 450 mm and a width of 30 mm. At each end to which the specimento be determined is applied, the paper is cut symmetrically to a lengthof 60 mm and a width of only 12 mm. The specimen will then be applied inthe form of a drop (approximately 0.01 ml) in the center at a distanceof 50 mm from the tapered end of the paper strip. The chromatographicpaper is then chromatographed in a suitable vessel. According to Rossel,a cylindrical glass vessel is expediently used that has a diameter of250 mm, a height of 500 mm, with a ground-in lid. In this glass vessel,the chromatographic strip is suspended in corresponding holdingarrangements in such a manner that it projects into the solvent which issituated at a height of approximately 10 mm on the bottom of the vessel.

Required Solutions

1. Methanol Solvents

Solution I: 133.3 g trichloroacetic acid and 30.0 ml 25% ammonia arefilled up to the amount of 1,000 ml with distilled water.

Solution II: 200 ml of 96% crystallizable acetic acid are filled up with800 ml distilled water.

For producing the methanol solvent, 120 ml methanol, 30 ml of solution Iand 10 ml of solution II are mixed with one another.

2. Spray Solutions

Molybdate Spray Solution

40 g sodium molybdate dihydrate and 50 g ammonium nitrate are dissolvedin distilled water and are filled up to the amount of 1,000 ml. Thissolution is then poured into 100 ml concentrated nitric acid.

Reduction Spray Solution

300 g sodium pyrosulfite and 10 g sodium sulfite and 2 g methol(N-methyl methyl amino phenol, Agfa Co.) are dissolved in 1,000 mldistilled water and, if necessary, are filtered.

Sodium molybdate solution for colorimetry

125 g sodium molybdate-dihydrate were dissolved in 1000 ml distilledwater.

Hydrazin Sulfate Solution for Colorimetry

0.3 g hydrazin sulfate are dissolved in distilled water. This solutionis always started fresh.

Perchloric acid (70%).

Implementation of Analysis

1 drop of approximately (0.01 ml) is applied to the chromatography paperas described above. The chromatograph strip will then be mounted in thevessel in such a manner that it reaches into the methanol solvent whichis situated at a height of approximately 10 mm on the vessel bottom. Thechromatographing expediently takes place at a constant temperature(ambient temperature) until the solvent front has risen to approximately5 cm below the upper edge of the chromatography paper.

This takes approximately 16 hours. Then the chromatogram is taken out ofthe vessel, is dried well in the drying chamber at approximately 60° to80° C., is then sprayed by means of a spraying bottle with the molybdatespraying solution to barely a uniform moisture, and is then again driedwell for approximately 5 to 10 minutes. Then spraying takes place in thesame manner by means of the reduction spraying solution and there isanother drying. The phosphates now appear on the paper as blue dots. Thephosphate dots are then cut out in such a manner that all phosphate isincluded but no superfluous paper is also cut. An approximatelypatch-sized piece without any phosphate is also cut out for a blank testand is subjected to the same treatment as all the others. (It isexpedient to let another chromatographic paper run along in parallel asa blank test during the chromatographing). The cut-out patches are thenfed into 50 ml--graduated flasks, are mixed with 4 ml of theconcentrated perchloric acid, and for the destruction of the paper, areheated without any placing of the ground-in stopper on the sand bath ata moderate temperature until, after a brisk boiling of the acid, whichhas become dark brown, a clarification has taken place and the acid asbecome white to light yellow. After diluting the acid solution toapproximately 25 ml, the graduated flasks are placed in a drying chamberfor 1 hour at 90° C. for hydrolysis.

The evaluation of the separated phosphates takes place colorimetrically.For this purpose, commercially available photometers may be used. 1 mlof the sodium molybdate solution are added to the cooled graduatedflasks for the purpose of colorimetry and 1 ml of the hydrazin sulfatesolution is added for the purpose of colorimetry as the reductionsolution. The graduated flasks are then placed in the drying chamber foranother 25 minutes at 90° C. The blank value will now appear in white tolight yellowish. After the cooling and the filling-up with distilledwater, the penetrability is determined. From the extinction of theindividual samples, relative to the sum of the extinctions, thephosphate proportion of the individual patches and thus the phosphatedistribution in percent can be determined according to

    n % P.sub.2 O.sub.5 =(100×E:ΣE).

Method d) Determination of the Fluoride Content in the Specimen by Meansof a Fluoride Electrode Which Responds Selectively to Fluoride beforeand after the Hydrolysis. The comparison of both numerical valuespermits the determination of the proportion of fluoride not bound tophosphorus in the produced monofluorophosphate.

The fluoride electrode 157205 of the firm Dr. W. Ingold AG,Urdorf-Zurich, Switzerland, which responds selectively to fluoride wasused.

Solutions required for carrying out the determination of fluoride:

Fluoride parent solution

aqueous solution with 2.2101 g sodium fluoride per liter. Such asolution contains exactly 1 mg fluoride per ml.

Tiron buffer solution

33.2 g. Tiron (Merck Co., Darmstadt) (Tiron=catechol disulfonicacid--3.5--disodium salt monohydrate) 102.06 g sodium acetate 58.44 gsodium chloride 15.01 g acetic acid (15.8 ml) filled up to 1 liter withdistilled water.

First, a calibration curve was established. For this purpose, 0.1 ml ofthe fluoride parent solution were pipetted into a 100 ml graduatedflask. Then a little distilled water was added and several granules ofhydroxyl ammonium chloride were added. Then phenol phthalein was addedand caustic potash solution was added until the solution had a slightlypink color. The pH-value was approximately 8. Then 10 ml of theabove-described Tiron buffer solution was added into the graduated flaskand the graduated flask was then filled up to the mark with distilledwater. The flask content was then filled into a dry 150 ml beaker(without rinsing). Then the fluoride electrode was dipped into thesolution and was moved back and forth in the solution. After theadjustment of the equilibrium, that is, after approximately 5 minutes,the display of a pH-meter, to which the fluoride electrode wasconnected, which is indicated in millivolt is read and recorded.

In the same manner, a 1 ml specimen and a 10 ml specimen were removedfrom the fluoride parent solution and were examined as described above.A calibration curve was established by entering the measured values inmillivolt against the content of fluoride in the respective usedsolutions in milligram on logarithmic millimeter graph paper.

Determination of the Fluoride Content in Sample Solutions before theHydrolysis:

The process product to be examined is dissolved in distilled water. Aprecisely determined volume, usually 1 to 50 ml, was removed from theobtained sample solution. The fluoride content of the sample was toamount to between 0.1 and 10 mg of fluoride. The removed sample wastreated as above; that is, it was pipetted into a 100 ml graduated flaskand was diluted with a little distilled water. Then several granules ofhydroxyl ammonium chloride were added and the pH-value was adjusted toapproximately 8 by means of caustic potash solution. Then 10 ml of theTiron buffer solution were added and the graduated flask was filled tothe mark with distilled water. Then, without rinsing, the content of theflask was poured into a dry 150 ml beaker. The fluoride electrode wasthen dipped in and moved back and forth. After 5 minutes, the measuredvalue was read in millivolt and was analyzed by means of the calibrationcurve.

Determination of the Fluoride Content after Hydrolysis:

An immersion heater distillation apparatus according to Dohr was used,as described in the advertising leaflet Glastechnische Information furLabor, Technikum und Betrieb No. 4 of the firm Glasapparatebau HerbertMiethke, Leverkusen. This apparatus comprises two vessels which arearranged inside one another. The outer vessel is a steam releasingdevice heated by an immersion heater, and the inner vessel is thereaction vessel. Both vessels are firmly connected with one another bymeans of a lid. A distillation top is placed on the reaction vessel. Thedistillation top, in turn, is adjoined by a handle cooler.

For the determination of the fluoride value after the hydrolysis, aprecisely weighed quantity of the sample (between approximately 0.3 andapproximately 0.4 g) is weighed into a 150 ml beaker. The sample wasdissolved in a little added distilled water; 2 ml silver sulfatesolution (=0.256 g silver sulfate) as well as 70 ml perchloric acid with70% in weight HClO₄ were added and the mixture was rinsed into theinterior vessel of the apparatus. Subsequently, a spatula of sea sand(approximately 3 to 4 g) was added. The outer vessel was then filled to3/4 of the volume with distilled water. A graduated flask which a litercontent, into which a little distilled water was added, was placed underthe handle cooler in such a manner that the cooler end dipped into thewater. Then, by means of the immersion heater, the water contained inthe outer vessel was slowly evaporated and the vapor was guided throughthe sample solution in the inner vessel. The heating was adjusted suchthat the boiling temperature did not exceed 108° C. The fluorine whichfrom the start existed as fluoride as well as the gradually hydrolyzingfluorine which is first bound to phosphorus are carried over with thewater vapor. After approximately 1 hour, the whole fluorine content ofthe sample was carried over by distillation, and the 1 l flask wasalmost full. The 1 l graduated flask was then removed from the cooler,the cooler end was rinsed off, and the flask was filled to the mark withdistilled water. 25 ml were removed from the flask content and werepipetted into a 100 ml graduated flask. The fluoride was then determinedin the manner described above; that is, distilled water and hydroxylammonium chloride were added; the pH-value was adjusted and the Tironbuffer solution was added.

By means of the calibration curve, the total content of fluorine in thesample can be determined from the measured millivolt value of the pHmeter.

From the comparison of the measured values of the sample before thehydrolysis and after the hydrolysis, the content of fluoride or offluorine, bound to phosphorus, in the respective sample can then becalculated.

As the result of the comparison of the fluoride content before thehydrolysis and after the hydrolysis, it can therefore be determined howmuch alkali fluoride is contained in the manufactured alkalimonofluorophosphate.

Method e) Ultimate Analysis

This method permits the determination of the atomic ratio of M:P:F.

EXAMPLE 1

112.0 g phosphoric acid (P-content approximately 27.69% whichcorresponds to an 87.46% acid and 1 mol H₃ PO₄) were placed in aplatinum vat. Then first 407.3 g caustic potash solution (27.55% inweight KOH, corresponding to 2 mol KOH) were added, and subsequently114.5 g hydrofluoric acid (19.22% in weight HF, corresponding to 1.10mol HF) were added. The initial atomic ratio of this preparation wastherefore K:P:F=2:1:1.10. The mixture was then heated to 350° C. Afterapproximately 1 hour, the reaction mixture was completely crystallized.It was then left for 1 hour at approximately 350° C.

Yield: 176.7 g.

EXAMPLE 2

The method of Example 1 was applied but the used amount of hydrofluoricacid was approximately 109.3 g (corresponding to 1.05 mol HF). Theinitial atomic ratio in the reaction mixture was thereforeK:P:F=2:1:1.05.

Yield: 177.1 g.

EXAMPLE 3

The method of Example 1 was applied but this time the used amount ofhydrofluoric acid was 104.1 g (corresponding to 1.00 mol HF), and theinitial atomic ratio was therefore K:P:F=2:1:1.00.

Yield: 176.3 g.

EXAMPLE 4

The method of Example 1 was applied but the used amount of hydrofluoricacid was 93.7 g (corresponding to 0.90 mol HF). The initial atomic ratiowas therefore K:P:F=2:1:0.90.

Yield: 175.9 g.

Characterization of the Products Obtained According to Examples 1 to 4

All four products are completely soluble in water. The phosphorus usedfor the reaction and the used potassium are completely present in theproducts. The distribution of the total phosphorus in the reactionproducts determined according to method c) resulted in the followingvalues:

                  TABLE 1                                                         ______________________________________                                        Result of the Quantitative Paper-Chromatographic                              Analysis of the Reaction Products of Examples 1 to 4                          Product                Phosphorus                                             According                                                                              Initial Atomic Ratio                                                                        Distribution (Atom %)                                  to Example                                                                             P:F           as PO.sub.3 F.sup.2-                                                                     as PO.sub.4.sup.3-                          ______________________________________                                        1        1:1.10        100        0                                           2        1:1.05        99         1                                           3        1:1.00          98.5       1.5                                       4        1:0.90        89         11                                          ______________________________________                                    

The phosphorus distribution determined by means of method b) is inagreement with the results of testing method c) indicated in Table 1.The testing according to method a) of the four reaction products provedthat no fluoride F⁻ was contained in the products. The Guinierphotographs of the products produced according to Examples 1 to 4illustrate that in each case crystalline dipotassiummonofluoromonophosphate K₂ PO₃ F was created. The purity of the productis particularly high when fluorine that is slightly leaner thanstoichiometric is applied.

Analysis according to method e) for the product obtained according toExample 3: K₂ PO₃ F (molar mass: 176.175)

                  TABLE 2                                                         ______________________________________                                        Ultimate Analysis for the Product According to Example 3.                              K           P      F                                                 ______________________________________                                        found      44.80         17.56  10.70                                         calculated 44.39         17.58  10.78                                         ______________________________________                                    

Table 2 indicates an atomic ratio of K:P:F=2.02:1.0:0.99. The results ofthe other methods of analysis are confirmed by this ultimate analysis.

EXAMPLE 5

First a solution was prepared of 174.2 g (1 mol) dipotassium hydrogenphosphate in 180 g distilled water. First 112.0 g phosphoric acid(P-content approximately 27.65% in weight, corresponding to 1 mol H₃PO₄) were entered into this solution; then 227.2 g caustic potashsolution (content of KOH approximately 49.4% in weight, corresponding to2 mol KOH), and finally 112.9 g hydrofluoric acid (HF-contentapproximately 39.0 in weight, corresponding to 2.2 mol HF). The mixturewas then heated to 320° C. until it is completely crystallized, and isthen held for another hour at this temperature.

Yield: 351.4 g.

The product consisted of pure crystalline K₂ PO₃ F, as demonstrated bythe methods of analysis a), b), c) as well as Guinier photographs.

EXAMPLE 6

Example 6 was carried in equipment according to FIG. 1.

A caustic potash solution containing 45% in weight KOH was charged intostorage vessel 1; a phosphoric acid containing 85.7% in weight H₃ PO₄was charged into storage vessel 2; and an aqueous hydrofluoric acidcontaining 40% in weight HF was charged into storage vessel 3. The valve4 was opened and 249.5 g of the caustic potash solution was allowed toflow into the mixing vessel 7. After the closing of valve 4, valve 5 wasopened and 114.5 g of the ortho-phosphoric acid solution was added.After the closing of valve 5, valve 6 was opened, and 53.5 g of thehydrofluoric acid solution was charged into the mixing vessel.

When a quantitative reaction to dipotassium monofluoromonophosphate isassumed, this corresponds to a molar ratio of water to the end productK₂ PO₃ F of 13.2:1 (corresponding to an aqueous solution of K₂ PO₃ Fwith 57.5% in weight of water).

The atomic ratio of K:P:F in the reaction mixture was 2:1:1.07. Thetemperature in the reaction mixture rose to approximately 70° C. Thefreshly prepared reaction mixture of a temperature of 70° C. was thenapplied to the aluminum plate 10 by way of the valve 8 and the pipe 9.It flowed uniformly, slowly and in a thin layer from the aluminum plate10 onto the roller surface of the roller 11 which was heated by means ofthermo-oil to a temperature of approximately 230° to 260° C. As soon asthe solution arrives on the roller surface, the water contained in itevaporated. It is assumed that now also the starting compounds reactwith one another while forming the desired monofluorophosphate. Theresulting vapor, which contained low concentrations of hydrogenfluoride, was discharged by way of a fume hood into a gas washer.

The roller was driven by means of a continuously controllable electricmotor with a chain. The reaction mixture remained on the roller surfacefor the duration of 2/3 of a rotation--approximately 30 to 40 seconds.The potassium monofluorophosphate which was formed and baked onto thesurface of the roller in the course of the drying operation was thenscraped off the surface of the roller by means of the blade 12. By wayof the conveyor belt 13, the product, which contained no more then 0.7%in weight of water, was placed in a drying oven and was dried there forthe second time for a duration of 20 minutes at 320° C. Then it wastransferred from the drying oven by way of the pipe 15 into a storagevessel 16.

The analysis was carried out according to method d) and e). The ratio offluoride before the hydrolysis to total contained fluorine, thusfluoride after the hydrolysis, showed that the product consisted ofapproximately 95% in weight of dipotassium monofluoromonophosphate.

                  TABLE 3                                                         ______________________________________                                        Ultimate Analysis for the Product of Example 6                                         K           P      F                                                 ______________________________________                                        Found      45.1          17.4   9.9                                           Calculated 44.39         17.58  10.78                                         ______________________________________                                    

The ultimate analysis confirms the result of the analysis according tomethod d).

We claim:
 1. A method of manufacturing an alkali monofluorophosphatecorresponding to the formula:

    M.sub.2 PO.sub.3 F                                         (I)

wherein M represents a cation of a Group I metal, said methodcomprising: forming a reaction mixture comprising alkali metal cationsM, phosphate P, fluoride F and water wherein the molar ratio of M:P:F is(2±0.1):(1±0.05):(1±0.1), and the molar ratio of water:P is at least1:1; heating said reaction mixture to a temperature of from 150° C. to400° C., whereby said alkali monofluorophosphate of formula (I) isformed as a reaction product while water is evaporated, and isolatingthe alkali monofluorophosphate reaction product.
 2. A method accordingto claim 1, wherein M represents an alkali metal cation selected fromthe group consisting of potassium, cesium and rubidium, and wherein themolar ratio of M:P:F in said reaction mixture is(2±0.05):(1±0.05):(1±0.1).
 3. A method according to claim 2, wherein themolar ratio of M:P:F in said reaction mixture is 2:1:(1 to 1.1).
 4. Amethod according to claim 1, wherein the molar ratio of water:P in saidreaction mixture is from 2:1 to 100:1.
 5. A method according to claim 4,wherein said reaction mixture is an aqueous solution.
 6. A methodaccording to claim 1, wherein said phosphate is introduced into saidreaction mixture in the form of phosphoric acid.
 7. A method accordingto claim 1, wherein M represents a potassium cation.
 8. A methodaccording to claim 7, wherein said fluoride is introduced into saidreaction mixture in the form of aqueous hydrofluoric acid.
 9. A methodaccording to claim 7, wherein said potassium cation is introduced intosaid reaction mixture in the form of a caustic potash solution.
 10. Amethod according to claim 1, wherein said reaction mixture is formed bymixing phosphoric acid, caustic potash solution and aqueous hydrofluoricacid.
 11. A method according to claim 10, wherein prior to said heatingstep said reaction mixture is maintained at a sufficient temperature toprevent precipitation of solids.
 12. A method according to claim 11,wherein said reaction mixture is heated in said heating step to atemperature of at least 200° C.
 13. A method according to claim 12,wherein said reaction mixture is heated in said heating step to atemperature in the range from 220° C. to 350° C.
 14. A method accordingto claim 1, wherein said heating step is effected by introducing thereaction mixture onto a surface of a heatable form and heating the form.15. A method according to claim 14, wherein said reaction mixture isapplied to the surface of a roller and is heated on the surface of theroller to a temperature of at least 150° C., and the reaction productwhich forms as water evaporates is removed from the surface of theroller by means of a self-supporting blade.
 16. A method according toclaim 15, wherein said reaction mixture is heated on the surface of theroller to a temperature in the range from 200° C. to 400° C.
 17. Amethod according to claim 16, wherein said reaction mixture is heated onthe surface of the roller to a temperature in the range from 220° C. to350° C.
 18. A method according to claim 1, further comprising subjectingthe isolated reaction product to a subsequent drying treatment at atemperature in the range from 280° C. to 400° C.
 19. A method accordingto claim 18, wherein the isolated reaction product is subjected to asubsequent drying treatment at a temperature in the range from 300° C.to 350° C.